NEW YORK (Reuters Health) - More than 10 percent of prescriptions in one Canadian province were for drugs not approved to treat the patient's condition, a new study finds. And many times, there was little evidence the drugs would work.

A medication is being used "off label" if a doctor prescribes it to treat a condition other than the one(s) Health Canada, the U.S. Food and Drug Administration or similar national regulatory agencies approved it for based on tests of safety and efficacy.

Dr. Tewodros Eguale, who led the new study, said doctors typically prescribe medications off-label when their patients fail to respond to other popular approved drugs or when they have a rare condition with few available treatments.

Eguale, from McGill University in Montreal, and his colleagues used data on every prescription written by Quebec physicians participating in an electronic health record network. The network is unusual in that it requires a doctor to state what the prescribed drug is intended to treat.

Between 2005 and 2010, 113 primary care doctors wrote more than 250,000 prescriptions for just over 50,000 patients.

Eleven percent of those prescriptions were considered off-label by the standards of the Health Canada drug database.

The researchers didn't have information on how well those drugs ended up working for the patients who took them. But they determined that four out of five off-label prescriptions didn't have strong evidence suggesting they were likely to be effective.

"Strong evidence" in this case included at least one controlled clinical trial -- considered the "gold standard" of medical research - showing the drug could help the patient's disorder.

Drugs meant to treat central nervous system conditions, like chronic pain, as well as infections and ear, nose and throat problems were most likely to be prescribed off-label.

The researchers also found that drugs approved by the Canadian government prior to 1981 were much more likely to be prescribed off-label than modern drugs.

Eguale told Reuters Health that not being approved for a specific condition doesn't mean a medication won't work, or that it's not safe.

Just as with newly-approved drugs, he said, "one has to be cautious. If you're giving it for off-label indications, you have to be careful about safety."

In an editorial accompanying the new study, published in the Archives of Internal Medicine, Dr. Patrick O'Malley wrote that unless pharmaceutical companies believe they can make a profit, they probably won't try to get new approvals for a drug even if its (off-label) use is widely accepted.

Getting a drug approved for specific conditions can be onerous and require lots of financial risk, according to O'Malley, an internist at the Uniformed Services University of the Health Sciences in Bethesda, Maryland.

The current U.S. research infrastructure would not be able to test whether every drug is appropriate for every known condition, according to O'Malley.

So there's no way to know whether the 11 percent of drugs used off-label in the new study is too much or too little, he said -- but there are things doctors can do to make sure they're using such drugs appropriately.

"Doctors should first be very clear about what evidence is available for that treatment, and they should be informing patients," he told Reuters Health.

Eguale and his colleagues found that physicians who said they practiced evidenced-based medicine were less likely to prescribe medication off-label.

"I think we need an informed physician, but we need an informed patient as well," he said.

The study did have certain limitations, the researchers note, including that the definition of "off-label" may have differed depending on how much of a drug was prescribed. And some apparently off-label use could have been explained by other conditions not included in a patient's electronic record.

The study also only looked at one province in Canada, which means the results may not be true for other areas of the country or in other countries.

But O'Malley said he suspects the results would not be that different in the U.S.

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